Conventionally, it has been considered that organic sludge requires utilization of auxiliary fuel to burn it. Applicant has conducted many studies on sludge incineration for many years and has invented a technology enabling sludge incineration utilizing less auxiliary fuel by a combustion method employing self-heating energy effectively, even of considerably low calorific sludge and/or even of sludge with high water rate as described in Japanese (application No. Sho-55-169165). FIG. 1 shows this incineration system.
Prior to the explanation of the present invention, the sludge incineration system in FIG. 1 is first described below.
Sludge having a water content reduced to 90 to 70 percent of the original water content by a mechanical dehydrogation method is contained in hopper 1. Forced supply means 2 such as a screw conveyor is provided at the bottom of the said hopper 1 to supply the sludge forcibly into heater 3. The sludge is heated by the said heater, the sludge is fluidized and then fed into drying furnace 4 having a fluidized sand bed. The dried products obtained in the said drying furnace 4 are drawn by suction of blower 11 and are led into separation means 5, and are separated to solidified sludge and dry gas is extracted from the sludge. The separated solid is fed into combustion furnaces by a constant quantity supply means such as screw conveyor 6. The combustion furnaces shown in FIG. 1 comprises a two stage sludge combustion system consisting of incomplete combustion furnace 7 and complete combustion furnace 7'; such an arrangement can largely reduce the NO.sub.2 generation rate.
The combustion air for these furnaces is, after being heated in air heater 14 arranged at the periphery of the complete combustion furnace 7', supplied to incomplete combustion furnace 7 and complete combustion furnace 7' as required. The gas produced from complete combustion furnace 7' is drawn by exhaust fan 10, is passed through heat exchanger 8 and filter 9 and is exhausted.
The gas separated by the said separation means 5 is pressurized by blower 11 up to 1000 through 3000 mm water column, and is elevated to a temperature as high as 200.degree. to 400.degree. C., and is sent through circulation passage 12. A required part of the gas is fed into drying furnace 4 having the said fluidized bed for drying, and the remainder is supplied to the said heater 3 for heating.
The gas heated in the same heater 3 is led into drain separation means 15 and, after the drain is removed from the system, is then led through gaseous fuel supply pipeline 16 into combustion furnace 7, for example, as gaseous fuel.
As such, when the heating gas mainly involving steam is heated through the sludge incineration system as shown in FIG. 1, after the drain is expelled out of the system by drain separation means 15, the heated gas is fed into combustion furnace 7. Therefore, the sludge can be burnt smoothly with a relatively small fuel replenishment rate and without harmful gas discharges.
Sewage sludge is taken up as an example. The sewage sludge solid obtained when the sewage sludge is dried by a drying furnace and is separated through a separation means has heating energy from 2000 to 5000 kcal/kg and the contents thereof are ash, in the amount of about 50 percent, and of the remaining portion, about 45 percent is volatile matter and about 5 percent is fixed carbon. The volatile matter can be made into a gaseous fuel by allowing almost all of its quantity to vaporize at an atmospheric temperature from 700.degree. to 450.degree. C. The required time is shorter if the temperature is higher, as shown in FIG. 2.
Moreover, applicant can reduce the sludge quantity to be disposed of by drying furnace 4 and combustion furnace 7, by means of a concentration apparatus which heats sludge and at the same time allows the evaporation of water contained in the sludge and the expulsion of the evaporated steam out of the system at the position of heater 3, instead of merely using heater 3 to simply heat sludge. Therefore, the sludge can be incinerated smoothly, even though drying furnace 4 and combustion furnace are both compacted.
The concentration apparatus shown in FIGS. 3 and 4 is the subject of applicant's Japanese application No. Sho 56-44743 corresponding to U.S. Pat. No. 4,388,875.
In FIG. 3, the sludge transferred by forced supply means 2 installed at the bottom of sludge containing hopper 1 is first fed into heating chamber 17 to which gas for heating is being supplied. The sludge is pressurized and heated in the said heating chamber 17 and is then injected into evaporation chamber 18 under a low pressure. The gas evaporated in the said evaporation chamber 18 is drawn through the suction pipeline having condenser 19, and the water included in the gas is removed from the system by condenser 19. A part of the sludge taken from the bottom of evaporation chamber 18 is supplied to drying furnace 4 having a fluidized sand bed but almost all of the sludge is recycled to heating chamber 17 and is treated during the circulation.
The heating gas fed into heating chamber 17, after being used for heating, is taken out of heating chamber 17, lowered in temperature a little and supplied as gaseous fuel, to combustion furnace 7 through supply pipe 16, after the drain is removed from the system by drain separation means 15.
In FIG. 4, the two sets of heating chambers 17 and evaporation chamber 18 are arranged in series, and by providing two or more such sets, sludge concentration can be carried out more effectively and can supply sludge with lower water content to drying furnace 4 having a fluidized sand bed in the next process step.